Certain properties of gypsum (calcium sulfate dihydrate) make it very popular for use in making industrial and building products, such as gypsum wallboard. Gypsum is a plentiful and generally inexpensive raw material which, through a process of dehydration and rehydration, can be cast, molded or otherwise formed into useful shapes. The base material from which gypsum wallboard and other gypsum products are manufactured is the hemihydrate form of calcium sulfate (CaSO4.½H2O), commonly termed “stucco,” which is produced by heat conversion of the dihydrate form of calcium sulfate (CaSO4.2H2O), from which 1½ water molecules been removed.
Conventional gypsum-containing products such as gypsum wallboard have many advantages, such as low cost and easy workability, although substantial amounts of gypsum dust can be generated when the products are cut or drilled. Various improvements have been achieved in making gypsum-containing products using starches as ingredients in the slurries used to make such products. Pregelatinized starch, like glue, can increase flexural strength and compressive strength of gypsum-containing products including gypsum wallboard. Known gypsum wallboard contains starch at levels of less than about 10 lbs/MSF.
It is also necessary to use substantial amounts of water in gypsum slurries containing pregelatinized starch in order to ensure proper flowability of the slurry. Unfortunately, most of this water eventually must be driven off by drying, which is expensive due to the high cost of the fuels used in the drying process. This drying step is also time-consuming. It has been found that the use of naphthalenesulfonate dispersants can increase the fluidity of the slurries, thus overcoming the water demand problem. In addition, it has also been found that the naphthalenesulfonate dispersants, if the usage level is high enough, can cross-link to the pregelatinized starch to bind the gypsum crystals together after drying, thus increasing dry strength of the gypsum composite. Thus, the combination of the pregelatinized starch and the naphthalenesulfonate dispersant provide a glue-like effect in binding the set gypsum crystals together. Trimetaphosphate salts have not in the past been recognized to affect gypsum slurry water requirements. However, the present inventors have discovered that increasing the level of the trimetaphosphate salt to hitherto unknown levels in the presence of a specific dispersant makes it possible to achieve proper slurry flowability with unexpectedly reduced amounts of water, even in the presence of high starch levels. This, of course, is highly desirable because it in turn reduces fuel usage for drying as well as the process time associated with subsequent water removal process steps. Thus the present inventors have also discovered that the dry strength of gypsum board can be increased by using a naphthalenesulfonate dispersant in combination with pregelatinized starch in the slurry used to make the wallboard.
The gypsum wallboards of the instant invention should be distinguished from acoustical boards or tiles that do not have face sheets. Also, the wallboards of the instant invention should be distinguished from acoustical boards or tiles that include polystyrene as a lightweight aggregate. Importantly, the aforementioned acoustical boards and tiles do not meet many ASTM standards that apply to gypsum wallboards. For example, known acoustical boards do not have the flexural strength required of gypsum wallboards including those of the present invention. Conversely, in order for acoustical boards or tiles to meet ASTM standards, it is required that an exposed surface of the acoustical boards or tiles have hollow voids or depressions that would be undesirable in a gypsum wallboard, and would adversely effect nail pull resistance and surface hardness properties.
Dust generation is a potential problem during the installation of all wallboard. When gypsum wallboard is worked, for example, by cutting, sawing, routing, snapping, nailing or screwing down, or drilling, substantial amounts of gypsum dust can be generated. For the purposes of the instant disclosure, “dusting” and “dust generation” means the release of airborne dust into the surrounding workspace during working of a gypsum-containing product, by, for example, cutting, sawing, routing, score/snapping, nailing or screwing down, or drilling the wallboard. Working can also generally include normal board handling, including dust produced on accidentally scraping and gouging the boards during transport, carrying, and installation. If a way could be found to produce a low density wallboard in which such dust generation is significantly reduced, this would represent a particularly useful contribution to the art.
Furthermore, if a way could be found to increase the strength of gypsum wallboard while lowering board weight, this also would be a useful contribution to the art. Air voids in known wallboard products have relatively thin walls in that the wall thickness between voids is about 20 to 30 microns, on average. If a new genre of gypsum wallboards could be provided with a microstructure comprising air voids with walls of enhanced thickness and a reinforced densified surface and therefore increased wall strength, an important and useful contribution to the art would be made. Additionally, if a way could be found to increase void size while increasing the thickness and surface density of the walls between the voids to produce a low density wallboard having enhanced strength and handling properties, this would represent yet another important contribution to the art.